This invention provides patterned, ion bombarded graphite field emission electron emitters, a process for producing them and their use in field emitter cathodes in flat panel display screens.
Field emission electron sources, often referred to as field emission materials or field emitters, can be used in a variety of electronic applications, e.g., vacuum electronic devices, flat panel computer and television displays, emission gate amplifiers and klystrons and in lighting.
Display screens are used in a wide variety of applications such as home and commercial televisions, laptop and desktop computers and indoor and outdoor advertising and information presentations. Flat panel displays are only a few inches thick in contrast to the deep cathode ray tube monitors found on most televisions and desktop computers. Flat panel displays are a necessity for laptop computers, but also provide advantages in weight and size for many of the other applications. Currently laptop computer flat panel displays use liquid crystals which can be switched from a transparent state to an opaque one by the application of small electrical signals. lIt is difficult to reliably produce these displays in sizes larger than that suitable for laptop computers or for operation over a wide temperature range.
Plasma displays have been used as an alternative to liquid crystal displays. A plasma display uses tiny pixel cells electrically charged gases to produce an image and requires relatively high electrical power to operate.
Flat panel displays having a cathode using a field emission electron source, i.e., a field emission material or field emitter, and a phosphor capable of emitting light upon bombardment by electrons emitted by the field emitter have been proposed. Such displays have the potential for providing the visual display advantages of the conventional cathode ray tube and the depth and weight advantages of the other flat panel displays with the additional advantage of lower power consumption than the other flat panel displays.
U.S. Pat. Nos. 4,857,799 and 51015,912 disclose matrix-addressed flat panel displays using micro-tip cathodes con structed of tungsten, molybdenum or silicon. WO 94-15352, WO 94-15350 and WO 94-28571 disclose flat panel displays wherein the cathodes have relatively flat emission surfaces.
Field emission has been observed in two kinds of nanotube carbon. structures. L. A. Chemozatonskii eti al., Chem. Phys. Letters 233, 63 (1995) and Mat. Res. Soc. Symp. Proc. Vol. 359, 99 (1995) have produced films of nanotube carbon structures on various substrates by the electron evaporation of graphite in 10xe2x88x925xe2x88x9210xe2x88x926 torr. These films consist of aligned tube-like carbon molecules standing next to one another. Two types of tube-like molecules are formed; the A-tubelites whose structure includes single-layer graphite-like tubules forming filaments-bundles 10-30 nm in diameter and the B-tubelites, including mostly multilayer graphite-like tubes 10-30 nm in diameter with conoid or dome-like caps. They report considerable field electron emission from the surface of these structures and attribute it to the high concentration of the field at the nanodimensional tips. B. H. Fishbine et:al., Mat. Res. Soc. Symp. Proc. Vol. 359, 93 (1995) discuss experiments and theory directed towards the development of a buckytube (i.e., a carbon nanotube) cold; field emitter array cathode.
R. S. Robinson et al., J. Vac. Sci. Technolo. 21, 1398 (1983) disclose the formation of cones on the surfaces of substrates under ion bombardment. The effect was reported for various substrate materials and were generated by simultaneously sputtering a surface at high energy while seeding it with impurity atoms deposited at low energy. They also disclosed the formation of carbon whiskers up to 50 xcexcm in length when a graphite substrate was ion-bombarded with impurities from a stainless steel target.
J. A. Floro et al., J. Vac. Sci. Technolo. A 1, 1398 (1983) disclose the formation of whiskers during relatively high current density ion bombardment of heated graphite substrates. The whiskers were disclosed to be 2-50 xcexcm in length and 0.05-0.5 xcexcm in diameter and to grow parallel to the ion beam. Simultaneous impurity seeding was reported to inhibit whisker growth. J. A. van Vechten et al., J. Crystal Growth 82, 289 (1987) discuss the growth of whiskers from graphite surfaces under ion sputtering conditions. They note that the whiskers of smallest diameter, characteristically about 15 nm, definitely appear to be different from either diamond or the scrolled-graphite structure found in carbon fibers grown by catalytic pyrolysis of hydrocarbons. Larger whiskers with diameters ranging from 30 to 100 nm were also observed to g""row in sputtering systems. The smaller diameter whiskers are constant in diameter along the length while the larger diameter whiskers may have a slight taper.
M. S. Dresselhauset al., Graphite Fibers and Filaments (Springer-Verlag, Berlin, 1988), pp. 32-34, disclose that filaments may be grown on several types of hexagonal carbon surfaces, but not on diamond or glassy carbon.
T. Asano et al., J. Vac. Sci. Technol. B 13, 431 (1995) disclose increased electron emission from diamond films which have been deposited on silicon by chemical vapor deposition, argon ion milled to form diamond cones and then annealed at 600xc2x0 C. These cones are formed if the diamond is in the form of isolated grains.
C. Nxc3xcitzenadel et.al., Appl. Phys. Lett. 69, 2662 (1996) disclose field emission from cones etched into both synthetic boron-doped diamond and silicon by ion sputtering.
S. Bajic et al., J. Phys. D: Appi. Phys. 21, 200 (1988) disclose a field emitter composite with graphite particles suspended in a resin layer.
R. A. Tuck et al., WO 97/06549, disclose a field emission material comprising an electrically conductive substrate and, disposed thereon, electrically conductive particles embedded in, formed in, or coated by a layer of inorganic electrically insulating material to define a first thickness of the insulating material between the particle and the substrate and a second thickness of the insulating material between the particle and the environment. The field emitting material may be printed onto a substrate.
There is a need for a process for readily and economically producing both small and large sized highly emitting field emission electron emitters for use in various flat panel applications.
This invention provides a process for producing a field emission electron emitter, which comprises:
(a) forming a layer of composite which comprises graphite particles embedded in a matrix material which comprises electrically insulating material, wherein the matrix material adheres to a substrate and to the graphite particles thereby affixing the graphite particles to one another and to the substrate and wherein the graphite particles are essentially completely surrounded by the matrix material, and
(b) bombarding the surf ace of the layer formed in (a) with an ion beam.
Preferably the ion beam is an argon ion beam and the argon ion beam has an ion current density of from about 0.1 mA/cm2 to about 1.5 mA/cm2, a beam energy of from about 0.5 keV to about 2.5 keV and a period of ion bombardment of at least about 15 minutes.
Preferably the electrically insulating material is glass and most preferably, a glass with a low softening point.
Preferably, when the layer of composite comprises graphite and glass, the process for forming the layer of composite on a substrate comprises screen printing a paste comprised of graphite particles and glass frit onto the substrate in the desired pattern and firing the patterned paste. For a wider variety of applications, e.g., those requiring finter resolution, the preferred process comprises screen printing a paste which further comprises a photoinitiator and a photohardenable monomer, photopatteining the dried paste and firing the patterned paste.
This invention also provides a process for producing a field emission electron emitter wherein the matrix material further comprises electrically conducting material. Preferably the electrically conducting material is silver or gold.
Preferably, when the layer of composite further comprises an electrically conducting material, the process for forming the layer of composite on a substrate comprises screen printing a paste comprised of graphite, glass frit and an electrically conducting material onto the substrate in the desired pattern and firing the patterned paste. For a wider variety of applications, e.g., those requiring finer resolution, the preferred process comprises screen printing a paste which further comprises a photoinitiator and a photo hardenable monomer, photopatterning the dried paste and firing the patterned paste.
This invention also provides a screen printable or coatable paste that can be used in the preferred process for embedding graphite particles in glass. The paste contains solids comprised of graphite particles and glass frit.
This invention also provides electron emitters produced by the process of this invention. These electron emitters and field emitter cathodes made therefrom are useful in vacuum electronic devices, flat panel computer and television displays, emission gate amplifiers, klystrons and lighting devices. The flat panel displays can be planar or curved.